COVER STORY C onstruction of a light rail line is a very large project in any city, and for Ottawa, the new LRT repre- sents the largest infrastructure project in the capital’s history. A great deal of local aggregate was used in constructing the new Confederation Line – for tunnels, tracks, stations and much more. To get the project off the ground, the City of Ottawa assembled a public-private partnership and secured financing commit- ments from the provincial and federal governments. The Rideau Transit Group (RTG) was selected amongst other bidders to oversee the project. Their selection was based on the strength of the group’s design, construction expertise and their financial proposal, as well as their track record in delivering major transportation projects on time and on budget, says RTG deputy project director Humberto Ferrer. RTG designed and began building the Confederation Line in 2013 and will main- tain it until 2038. Equity partners in the project are ACS Infrastructure Canada Inc., SNC-Lavalin and EllisDon. The Design- Build Joint Venture (DBJV) is made up of Dragados Canada Inc., EllisDon and SNC-Lavalin. In a project of this scope, many compa- nies are involved, and this includes four Ottawa-area businesses contracted for aggregate. As well, during the initial plan- ning and development phases, Delcan Corporation produced a preliminary geotechnical program as part of the environmental assessment, and Golder Associates performed a detailed geotech- nical and hydrogeological investigation as part of the preliminary engineering program for the City of Ottawa. Ferrer says an integral part of RTG’s proposal was the approach it planned to use in the construction of the tunnel under Ottawa’s downtown core. Among the factors needing thorough evaluation were the specific soil conditions of the down- town core and proximity to the Rideau Canal (a UNESCO World Heritage Site). LIMITED DISTURBANCE Because it provides many important bene- fits, the Sequential Excavation Method (SEM) was used in mining the LRT tunnel, which started in late 2013. SEM not only provides a high degree of flexibility to adjust the excavation approach and support measures for various soil and rock condi- tions, it also reduces the risks of tunnelling construction and minimizes longer-term disruptions at the surface. The use of SEM meant that only three mining access points were required for Ottawa’s LRT, drastically minimizing road closures and detours. These points were a central shaft (measuring 22.5 metres long by 12 metres wide and 18 metres deep) and two portals: East (75 metres long by 12 metres wide) and West (90 metres long and 12 metres wide). “RTG used shoring systems and then excavated rock in the West portal and the shaft,” Ferrer reports. “In the East portal, because the section of rock requiring excavation was much less than in the other areas, a roadheader was used to excavate rock.” Ferrer says using smaller, quieter road- headers to dig the tunnel (instead of a large tunnel-boring machine) greatly limited noise and vibration for surrounding build- ings. He points out that much of the WINTER 2018 AVENUES 9